Chlorinated paraffins are classified as chlorinated hydrocarbons that have the general formula C.sub.x H.sub.(2x-y+2) Cl.sub.y. They were first prepared in 1858 by P. A. Bollcy. Significant commercial uses did not develop until the early 1930's when they were first used for fire-retardant and water-proof canvas material, and in the metal working industry as extreme pressure additives for lubricating oils. The raw materials used in the chlorination reaction consist of petroleum fractions such as normal paraffins, at least 90% linear, and wax fractions averaging as many as 30 carbon atoms. While there are a number of raw materials available, those used for the production of chlorinated paraffins fall into three categories: (1) a C.sub.12 fraction that normally includes C.sub.9 -C.sub.4 hydrocarbons; (2) a C.sub.15 fraction that normally includes C.sub.13 -C.sub.17 hydrocarbons; and (3) a C.sub.24 fraction that normally includes C.sub.20 -C.sub.30 hydrocarbons. The selection of a particular raw material is dependent on the desired properties of the final chlorinated paraffin. Isoparaffins (usually&lt;1%), aromatics (usually&lt;100 ppm), and metal contamination are kept as low as economically feasible since their presence results in products with undesirable properties.
In the United States, approximately 50% of the chlorinated paraffins, are used as extreme pressure lubricant additives in the metal working industry. About 25% are used in plastics, including fire retardant and water repellent coated fabrics. The remainder are used in rubber, caulks, and sealants.
Commercial chlorinated paraffins have a range of between .about.20-75% chlorine content. The bulk of the manufactured products fall within the 40-70% chlorine range. Chlorine content as used herein, refers to the amount of chlorine chemically fixed or bonded to the paraffin and not to any free chlorine or the chlorine content of any residual chlorinated solvent remaining in the chlorinated hydrocarbon material. The important physical properties of the chlorinated paraffins include viscosity, solubility, color, and thermal instability. For a given paraffin, increasing chlorine content increases viscosity and specific gravity. With chlorinated paraffins of the same chlorine content, lower viscosities are observed for the lower molecular weight paraffins.
The techniques for chlorinating C.sub.20-30 paraffins has long been known. When a product exhibiting a relatively low degree of chlorination is desired, e.g., up to about 40% chlorine by weight, the use of a solvent may generally be avoided. By the elimination of a solvent from the system, the effective capacity of the reaction vessel is increased and product recovery techniques are simplified. When higher chlorine contents, eg. about 55-73% by weight, the C.sub.20-30 paraffin hydrocarbon wax, is typically dissolved in an organic solvent which is inert, in the sense that it does not interfere with the desired reaction and which also serves as a diluent during the chlorination reaction. Since the chlorination reaction is exothermic, the intimate presence of the solvent is helpful in maintaining the desired reaction temperature. A solvent may be selected having the requisite boiling point so that heat from the reaction zone may be withdrawn as the solvent is refluxed. While the use of a solvent has been described, paraffins of C.sub.10-13 and C.sub.13-17 do not require a solvent, even up to 65-70% chlorine levels. However, the C18-30 product obtained by neat chlorination is a dark colored product with poor stability when chlorine contents of over 65% are attempted.
Preferred solvents are halogenated C.sub.1 or C.sub.2 hydrocarbons. For instance, carbon tetrachloride, chloroform, pentachloroethane, perchloroethylene, 1,1,1-trichloroethane, 1,1,2-trichloroethane, and ethylene dichloride may serve this role. U.S. Pat. No. 3,948,741 teaches that a diluent-solvent should possess enough chlorine or fluorine moleties to render it substantially inert, i.e., toward further chlorination at the conditions of the reaction. The preferred diluent-solvent was indicated to include hydrocarbons wherein at least about 30% of the hydrogen atoms were replaced by chlorine or fluorine. Among the preferred diluent-solvents were perfluorinated or perchlorinated (or perfluoro-perchloro) alkanes including normal and branched-chain alkanes.
The '741 patent indicated that the diluent-solvent could include inert substituents. Certain substituents, which were inert as above defined, were indicated to in fact, increase the selectivity of the reaction, i.e., the synthesis of 1-chloro normal alkanes. Examples of such insert substituents included hydroxy, nitrile, carboxylic acid, carboxylic acid ester, and ketone functionalities. It was additionally taught that aromatic rings, sulfides, nitro groups, amines and phosphines when present as substituents in the diluent solvent (used alone) caused a decrease in the desired selectivity, and therefore it was preferred not to use diluent-solvents alone which contained these moieties. The particularly preferred solvent is carbon tetrachloride. Analogous chlorofluoroalkanes, hexachlorobutadiene, and many other solvents suitably inert under the reaction conditions may also be used as will be apparent to those skilled in the art. Additional examples of such would include: trifluoroacetic acid, trichloroacetic acid, 2,2,2-trichloroethanol, trichlorofluoromethane, hexachloroacetone, trichloroacetonitrile, pentachloroethylene, ethyl heptafluorobutyrate, ethyl trifluoroacetate, methylene dichloride, 1,2-dichloroethane, 1,1,2-trichloro- 1,2,2-trifluoroethane, 1,2,3,4-tetrachlorobutane, octachloropropane, heptachloroisobutanes (mixed isomers), 1,1-dichloro-1,1,2,2-tetrafluoroethane, and hexachloroethane.
Chlorinated paraffins are typically produced by passing chlorine gas into a liquid paraffin directly, with or without a solvent, or into a solubilized paraffin solution. The chlorination reaction is a typical substitution reaction and hydrogen chloride is formed as a by-product. Because of the corrosive nature of the hydrogen chloride reaction by-product and chlorine gas reactant, special care must be given to materials of construction. Ultraviolet light is often used to promote chlorination, especially at higher chlorine contents.
Chlorine feed rates and reaction temperatures differ slightly among the different producers. The reaction is exothermic and temperatures are usually kept at 50.degree.-100.degree. C. The following techniques may be employed to moderate the exothermic portion of the reaction and to maintain the desired temperature: (1) refluxing the solvent with the concomitant removal of heat from the reaction mixture; (2) cooling the walls of the reactor; and (3) controlling the rate of chlorine introduction. The exact temperature selected for optimum results will be influenced by (1) the boiling range of the solvent; (2) the reaction pressure on the vessel used during the chlorination; and (3) the relative concentration of paraffin in the solvent. Manufacture of resinous chlorinated paraffins (70% chlorine content), generally requires the use of a solvent, such as carbon tetrachloride to dissolve the intermediate chlorinated paraffin and to allow the chlorine level to increase to the 70% level where the product is a definite solid with sufficient hardness to be ground and processed as a powder after solvent stripping. However, it is possible to prepare a 70% chlorinated resin without a solvent. But this procedure requires higher temperatures, e.g., 135.degree. C., and takes a long time due to high viscosity. The final product is typically darkly colored, with poor stability.
Carbon tetrachloride however, is undesirable because it is believed to cause cancer and has been implicated in potential ozone layer depletion. Virtually all homologous solvents that could be used as a substitute for carbon tetrachloride have similar problems. If they are significantly higher in molecular weight, their higher boiling point makes it difficult to remove the solvent from the chlorinated wax product.
Thus, in general, highly chlorinated resins are generally prepared by starting with a C.sub.20 -C.sub.24 essentially linear material, which under modest pressure (e.g. 5-10 psig), if any pressure is used at all, is chlorinated using neat chlorine, producing a highly viscous oil, of roughly 50% chlorinated 5paraffin. This material is now soluble in, for example, CCl.sub.4, in which a roughly 50/50 mixture of chlorinated solvent and partially chlorinated paraffin is refluxed and chlorine is introduced. This exothermic reaction is limited in rate to the cooling capacity of the production equipment. HCl is a by-product of the reaction. The mixture is reacted with chlorine until approximately 70% chlorine containing chlorinated paraffin resin is produced. The bulk of the solvent CCl.sub.4 is distilled off. Residual CCl.sub.4 is however, quite difficult to remove. In one method, CCl.sub.4 is removed by melting the chlorinated paraffin resin at about 140.degree. C., thereby reducing the level of residual CCl.sub.4 to .about.1-2%. This amount can be further reduced to about less than 0.1% by using vacuum stripping. Upon chilling the chlorinated paraffin resin, the product can be reduced in particle size by using a hammer mill.
However to date, there still exists the need to provide a chlorination process which can be used to manufacture highly chlorinated resins without using ozone-depleting organic solvents, especially chlorinated solvents which are difficult to remove from the product in finished form.